The present invention relates to steam generators and their heat exchange and gas clean-up adjuncts, e.g. tubular or regenerative air heaters and electrostatic or mechanical precipitators, respectively, and more particularly to the protection of gas-side surfaces during shutdown of the steam generator.
Most flue gases contain sulfur and the expose metal parts of the steam generator and its heat exchange and gas clean-up adjuncts must be kept above the dew point, i.e., the temperature at which water vapor begins to condense. The condensation of air entrained vapor is generally recognized as a major contributing factor to out of service corrosion. During shutdown of a steam generator, it is known that ambient air will enter the gas-side and result in cold spots where the water vapor in the air could condense. The condensation of the vapor is promoted when the gas-side is exposed for extended periods to relatively cold air thereby maintaining the heat transfer surfaces in a moist condition. The condensate combines with sulfur bearing deposits to form acids which corrode the exposed gas-side metal surfaces. It is an object of the present invention to provide an arrangement whereby the gas-side surfaces are kept above the dew point at all times while the steam generator and its heat exchange and gas clean-up adjuncts are out of service.